Vital sign monitoring system, extensively applied to healthcare and fitness equipment, can track health and exercise of a biological subject by capturing and analyzing the changes in vital signs.
In general, there are two types of conventional vital sign monitoring system, contact type and non-contact type. The contact type vital sign monitoring system has to contact the subject's skin for a long time for accurate detection of the subject's vital signs, and the subject's moving range may be limited by the wires connected to the contact type vital sign monitoring system, causing the subject to feel uncomfortable during monitoring. In contrast, the non-contact vital sign monitoring system transmits wireless signals to the subject, the subject's vital signs can be detected according to the Doppler shift of the reflected signals. However, the reflected signals caused by body movement of the subject will also cause Doppler shift to affect the vital sign monitoring. Therefore, how to cancel the random body movement effect is a key point for developing the non-contact vital sign monitoring system.
Please refer to U.S. Pat. No. 8,721,554, a prior art of non-contact vital sign monitoring system able to cancel the random body movement effect is described. The system configures two continuous-wave (CW) radar apparatus at opposite sides of the subject, and transmits wireless signals to the subject and receives reflected signals from the subject via the antennas, wherein the subject's body movement direction is opposite relative to the antennas of the two CW radar apparatus, but the subject's vital sign (e.g. respiration and heartbeat) movement direction is identical relative to the antennas of the two CW radar apparatus. For this reason, the system can use signal post-processing to cancel the random body movement effect on the detection of vital signs of the subject. However, the system is limited in configuration because the two CW radar apparatuses have to configure at opposite sides of the subject. The subject's vital signs are therefore difficult to detect by the system when the subject stands against a wall or lays on a bed.
Please refer to U.S. Pat. No. 8,754,772, a prior art of non-contact vital sign monitoring system is disclosed previously by the present inventors. Two self-injection locking (SIL) radar apparatuses configured at opposite sides of the subject transmit wireless signals to the subject using the same gain antennas. In the same principle, the subject's body movement direction is opposite relative to the antennas of the two SIL radar apparatus, and the subject's vital sign (e.g. respiration and heartbeat) movement direction is identical relative to the antennas of the two SIL radar apparatus. Hence, the two SIL radar apparatuses can be mutual-injection locked to each other to real-time cancel the random body movement effect on the detection of vital signs of the subject. However, the system is also limited in configuration because the two SIL radar apparatuses need to be placed at opposite sides of the subject.